1. Field of the Invention
This invention relates to an improved method and composition for inhibiting shrinkage in cementitious systems during setting and hardening and, more particularly, to a method and composition for controlling the contraction of such cementitious systems through the addition of gas generating agent additives.
The term "cementitious systems," as used herein, is intended to include compositions which generally possess the characteristic of hardening under water and includes, for example, settable hydraulic cement, hydraulic lime, gypsum and like materials, as well as mixtures of the foregoing with aggregates and water such as concrete, mortar, grout and products made therefrom.
2. Description of the Prior Art
Various methods and means for inhibiting shrinkage of hydraulic cement mixtures during setting and hardening have previously been suggested. These methods have included the addition, to such mixtures, of gas delivering agents such as aluminum powder and expansion agents such as iron filings. These procedures have been impractical because of, among other reasons, lack of adequate control of expansion and production of a non-uniform product. It has also been found that certain unique materials can eliminate shrinkage in concrete due, it is theorized, to the release of entrapped gas from porous particulate materials upon adsorption of water from the cementitious system. Thus, materials such as fluid coke, a combination of fluid coke and delayed coke, which are byproducts of the petroleum industry, and porous particulate materials, such as so called industrial adsorbents, have been used with various types of cementitious mixtures to successfully inhibit shrinkage. See for example U.S. Pat. Nos. 3,503,767; 3,519,449; 3,794,504; 3,890,157; and Re. 26,597.
U.S. Pat. No. 3,591,394 describes the drawbacks associated with the usual gas delivering and expansion compositions involving metal powders when added to cement to compensate shrinkage. The patent proposes the use of a nitrogen delivering compound which produces an expansion or formation of pores in the concrete material and states that while nitrogen delivering compounds have been used in expanding agents for rubber products such has been accomplished at high temperatures only. Nevertheless, U.S. Pat. No. 3,591,394 requires the addition, to concrete, of an activator together with a nitrogen delivering hydrazine derivative or diazonium compound to chemically cleave off nitrogen. Not only does the need for an activator introduce another constituent to be supervised and the difficulties attendant therewith but it is believed that certain activators can have a deleterious effect on the cement mixture. For example, soluble borates may retard cement set and lower its strength and great care is necessary in using retardant since setting and hardening of cement can be totally inhibited thereby. Thus, the use of a perborate activator may present drawbacks.
In addition, I have found that the use of a hydrazine derivative such as benzene sulfonyl hydrazide either does not produce expansion without an activator or produces an expansion which fades before setting occurs so that the cement sags below placement volume before setting, when measured by the ASTM method of test for early volume change of cementitious mixtures, ASTM C827.
The use of hydrogen peroxide or sodium peroxide to foam concrete is attendant with the same drawbacks arising from the use of metal powders; namely, a lack of effective control due to almost immediate reaction to generate gas.
The foregoing demonstrate that the successful utilization of certain materials as shrinkage inhibitors to control contraction in setting cementitious systems is unpredictable. For example, fluid coke is a carbonaceous material and therefore its success as a shrinkage inhibitor without having deleterious effects on the cementitious system is surprising in view of the general attitude of the art that such materials should not be added to concrete. Also, the expansion arising from metal additives is largely uncontrollable so as to prevent their effective utilization where control, produce uniformity and repeatability of results is desired. In addition, U.S. Pat. No. 3,591,394 demonstrates that, while expanding agents may be known to be used in one industry, for example, in the plastics industry as expanding agents for rubber products, the parameters of operation in different industrial fields of application are frequently of such difference that one would not be led to believe that additive constituents would be interchangeable in the two.